In recent years, for portable information devices and many other information storage devices, an increase in the capacity of a memory has contributed not only to improved performance but also to environmental issues given a reduction in carbon dioxide emissions per bit.
A Magnetic Random Access Memory (MRAM) is a nonvolatile memory that uses magnetism, thereby reducing power consumption in standby mode. The MRAM is expected to help personal computers save more power.
Increases in the capacity of hard disk drives or solid-state memories have been achieved by increasing the functionality of an optical exposure system to reduce the writing/reading size of one bit. In recent years, it has become possible to write a finer pattern with the use of a KrF excimer stepper, ArF excimer stepper or immersion ArF excimer stepper.
In order to further decrease the size of one bit and increase the capacity of a device, the International Technology Roadmap for Semiconductors (ITRS) focuses on the next-generation mask formation technologies, such as an exposure device and direct-writing electron beam exposure device that use an EUV (Extreme Ultra Violet) ray, a technique, called double patterning, of carrying out shift exposure twice, and a technique of transferring a thin film formed on a side wall as a mask.
According to the sidewall transfer (SWT) method, a formative layer (sidewall film) is formed to form an uneven pattern on the periphery of a guide (core) pattern that is formed by exposure or other methods. The formative layer (sidewall film) is trimmed by etching to selectively leave the layer on the side wall of the core. Then, the core is removed to form the uneven pattern of the formative layer. Since the sidewall transfer technique uses the thin film formed on the side wall as the width of a mask, the limit width dimension is considered to be at atomic levels in principle.
According to the sidewall transfer method, it is desirable that the mask pattern formed be in the shape of a rectangle whose angles are substantially right angles; it is necessary to keep the corners from being cut off. High etching anisotropy is required to form a rectangular mask pattern; dry etching is mainly used. Therefore, as for a dry etching condition, the formation of a sidewall protective film whose etching rate is slow compared with the sidewall film leads to an excellent rectangular quality.
Dry etching has both a chemical etching component, which utilizes a chemical reaction, and a physical etching component, which utilizes collision energy of particles. For the chemical etching component, etching proceeds isotropically. For the physical etching component, etching proceeds anisotropically. When etching proceeds isotropically, high anisotropy cannot be obtained.